Garnet is a cubic silicate mineral represented by the chemical composition M2+3M3+2Si3O12 (wherein M2+ is Mg, Ca, Mn, or Fe, and M3+ is Al, Cr, or Fe). Garnet-type compounds having crystal structures similar to the structure of garnet are not limited to silicates, but all of the M2+, M3+, Si4+ ion positions in the crystal structure can be replaced with ions of various valencies. Accordingly, various garnet-type compounds having a structure similar to that of garnet exist. Among chemically synthesized garnet-type compounds, some are widely used in industry. An example of a well-known chemically synthesized garnet-type compound is yttrium aluminum garnet (Y3Al2Al3O12), which is widely used as an industrial laser material. Yttrium iron garnet (Y3Fe2Fe3O12) is used as a magnetic material or a faraday rotor.
In recent years, a specific garnet-type compound has been attracting attention as a potential material that could contribute to the development of the battery industry. The start of this was Non-patent Literature (NPL) 1, which reports that a synthesized garnet-type compound Li5La3M2O12 (wherein M is Nb or Ta) is a solid electrolyte material having remarkable lithium ion conductivity and Non-patent Literature (NPL) 2, which reports that another garnet-type compound Li7La3Zr2O12 (hereinafter abbreviated as “LLZ”) is a solid electrolyte material having remarkable lithium ion conductivity. These compounds contain an excess of Li, compared with an ideal garnet-type structure. Having this specific crystal structure is considered to be one of the reasons why these compounds exhibit high lithium ion conductivity in a solid state. In particular, LLZ containing Zr as a component has a high lithium ion conductivity of 10−4 S/cm at room temperature.
Further, techniques are proposed regarding compounds analogous to LLZ, which are produced by adding various additional elements to LLZ and also have a garnet-type structure (see, for example, PTL 1 and PTL 2). Compared to normal LLZ, such compounds analogous to LLZ are reported to have increased stability of the garnet-type structure, higher ionic conductivity, etc., due to the effects of the additional elements. LLZ and compounds analogous to LLZ are hereinafter collectively referred to in this specification as “LLZ garnet-type compounds.”
LLZ garnet-type compounds, which have high lithium ion conductivity and high electrochemical stability against lithium metal, are promising candidates as solid electrolyte materials for all-solid lithium secondary batteries. All-solid lithium secondary batteries, which comprise a non-flammable solid electrolyte, are next-generation secondary batteries that provide the ultimate level of safety. Research and development of materials and devices for putting such new-generation secondary batteries into practical use are actively being conducted.
When an LLZ garnet-type compound is to be applied to all-solid lithium secondary batteries, producing an LLZ garnet-type compound in the form of fine particles is desirable. This is because when a solid electrolyte material is to be used as a lithium ion conduction aid in an electrode layer, a solid electrode material that is in the form of fine particles in a battery manufacturing process is considered to be suitable to ensure a sufficient contact interface with an electrode active material. Such a fully ensured contact interface between the solid electrolyte material and the electrode active material satisfactorily establishes ion conducting pathways to thereby increase the utilization rate of a positive electrode active material, thus providing an all-solid lithium secondary battery as a storage battery with high energy density.
In view of recent social demands for lowering the price of secondary batteries, establishing a method for producing a solid electrolyte material, which is a main component of all-solid lithium secondary batteries, at a low cost with high productivity is also considered to be of importance to put all-solid lithium secondary batteries into practical and common use.
From these viewpoints, with attempts to improve the above properties of LLZ garnet-type compounds, various techniques for methods of producing LLZ garnet-type compounds have been proposed (see Non-patent Literature (NPL) 2 and Patent Literature (PTL) 1 to PTL 4).